1. Field of the Invention
This invention relates to heat-treating glass sheets while conveyed on a roller conveyor.
It is known to temper flat glass by heating a series of glass sheets while conveying the latter in a horizontal plane on spaced, rotating, parallel conveyor rolls through a tunnel-type furnace having an entrance end and an exit and continuing to convey the heated sheets on additional spaced, rotating, parallel conveyor rolls through a cooling station where the sheets are chilled rapidly by blasting their opposite surfaces with jets of tempering medium such as blasts of cold air. In order to achieve a desired degree of temper, it is necessary to heat the glass to a temperature sufficiently high (corresponding to a viscosity of 10.sup.13.3 poises for ordinary soda-lime-silica glass) to permit the tempering medium to impart the desired temper. It was difficult in the prior art to raise the temperature of the glass in the furnace to a temperature sufficient for tempering without inducing in the glass a tendency to develop a defect known as roll ripple distortion. Roll misalignment and roll spacing within the furnace were considered to be the major factors causing roll ripple distortion.
In the past, it was proposed to have the tempering apparatus as close to the exit of the furnace as possible, thus reducing the time in which the glass cooled before impinging blasts of tempering medium could be applied thereon. However, this proposed solution was accompanied by cold air entering the exit end of the furnace where the temperature should be highest, thus disrupting the desired temperature gradient of glass sheets along the length of the furnace from a minimum at the entrance end of the furnace to a maximum at the furnace exit, and also disrupting the desired temperature pattern across the width of the furnace, particularly in the vicinity of the furnace exit where the temperature pattern in the glass should be controlled most closely. One of the solutions to this problem is to space the cooling or quenching station from the furnace exit sufficient distance to minimize this problem.
The roller hearth conveyors used in the past have included conveyor rolls having their upper surfaces aligned as precisely as possible throughout the entire length of the furnace to provide spaced, rotating lines of support for moving flat glass sheets in as nearly a perfectly aligned plane as possible. In addition, prior art roller hearth conveyors have included relatively small diameter rolls relatively closely spaced near the exit end of the furnace in order to minimize roll ripple distortion. However, prior to the present invention, the portion of the roller conveyor beyond the furnace exit was either subjected to less severe maintenance than the rolls within the furnace or were ignored altogether.
Since it was considered necessary to apply tempering medium such as air blasts between adjacent conveyor rolls downstream of the furnace exit onto the opposite glass sheet surfaces and to provide paths for the impinging air to escape between the same rolls, the rolls located downstream of the furnace were more widely separated than the rolls within the furnace immediately upstream of the furnace exit. An alternative prior art structure for the roller conveyor portion beyond the furnace exit provided offsetting discs supported on relatively small diameter shafts spaced apart sufficient distances to provide space between adjacent disc axially of each shaft and between discs on adjacent shafts for the application and escape of tempering medium between said discs.
Unfortunately, the prior art resigned itself to the fact that increasing the separation between adjacent rollers or between adjacent discs that supported the hot glass immediately beyond the furnace was a cause of surface irregularity that worked against the beneficial result of surface smoothness that close roll spacing within the hottest portion of the furnace was desired to attain. In addition, the prior art did not appreciate that some latitude of misalignment is permissible in the parallelism of the conveyor rolls located in spaced relation to and upstream of the furnace exit where even though one or more misaligned rolls imparts a pattern of irregularity in the bottom glass surface, it could be removed by rolling the surface over properly aligned rolls at higher temperature.
Furthermore, while the prior art may have ignored altogether the maintenance of near-perfect alignment of all rolls located downstream of the furnace exit, it did not appreciate fully that only those conveyor rolls disposed a sufficient distance downstream of the furnace exit to provide rotating support for glass sheets in downstream locations where the glass was cooled sufficiently to be rigid enough to avoid distortion on engaging said downstream rolls could be ignored from a periodic maintenance program. Hence, considerable time and expense was wasted in servicing all the conveyor rolls when less time and expense was needed to maintain the conveyor in adequate running condition by careful maintenance and repair of the conveyor rolls located in a critical portion only of the conveyor, the location of the critical portion in the prior art erroneously being located entirely within the furnace near the furnace exit.
2. Description of the Prior Art
U.S. Pat. No. 3,806,331 to Bezombes discloses a system for conveying glass sheets through a tunnel and through a quenching station disposed immediately outside the exit of the tunnel. Conveyor rolls having substantially the same diameter are provided throughout the length of the furnace and of the quenching station. The conveyor rolls in the hotter part of the furnace are almost contiguous to reduce the space between the lines of support and prevent sagging between the adjacent rollers. However, the space between the rollers immediately beyond the exit of the furnace was increased to provide space for air blasts to be applied against the opposite surfaces of the flat glass and to escape after impinging on the opposite glass sheet surfaces. The cooling blasts of air applied immediately beyond the furnace exit in Bezombes had to be shielded from the furnace by several furnace shielding means. Furthermore, the need for the provision of relatively wide spacing between rolls immediately outside the furnace exit results in a certain amount of roll ripple distortion in the resulting tempered flat glass sheets.
U.S. Pat. No. 2,144,320 to Bailey, U.S. Pat. No. 3,396,000 to Carson et al. and U.S. Pat. No. 3,454,338 to Ritter disclose conveyor rolls comprising shafts of small diameter that support a series of spaced collars or discs which are offset from roll to roll to provide additional space for the application and escape of air blasts in a cooling station of glass tempering apparatus. Unfortunately, the heat-softened glass sheets tend to sag between these supporting collars not only in the direction of glass movement, but also between spaced collars mounted on the same shafts in the direction transverse to glass movement in the axial direction of the rolls. Hence, roll ripple distortion in glass sheets treated on such apparatus is complicated because two distortion patterns are possible, one parallel to the conveyor rolls or shafts and another transverse to the first distortion pattern.
U.S. Pat. No. 2,140,282 to Drake discloses a roller conveying system for flat glass sheets in which the conveyor rolls are relatively widely spaced in a primary heating zone of a furnace where the glass is initially heated, and relatively closely spaced toward the exit end of the furnace where the glass temperature is much hotter than in the first stage of the furnace. Immediately beyond the furnace exit, the conveyor rolls comprise a series of horizontal shafts of relatively small dimension, each carrying a plurality of spaced short cylindrical discs with the discs on each shaft disposed in overlapping relation and offset from the discs of adjacent shafts. This construction also permits some roll ripple distortion both in the longitudinal direction from shaft to shaft and in a transverse direction from disc to disc mounted on any shaft.
Furthermore, none of the patents enumerated appear to recognize the economic benefit of concentrating primarily on aligning and maintaining in close alignment only those conveyor rolls in a critical portion of the conveyor. Neither does any of the prior art patents recognize the position of the critical portion of the conveyor.
Despite the many patents existing in the glass sheet conveying art, there still remained need for improving the appearance of the surfaces of glass sheets which contain poor optical properties associated with the local deformation caused in the glass by spaced, rotating supports that were misaligned.